Conventionally, in a manufacturing line of pneumatic tires, various characteristics regarding tire uniformity (e.g., RFV, LFV, conicity and the like) are checked before shipment in order to suppress vibration of tires during rolling. When measuring the characteristics regarding tire uniformity, a pneumatic tire is mounted on a measuring rim. For example, vertical rigidity distribution of the tire can be obtained by rotating the rim together with the pneumatic tire on a drum of a uniformity measurement device so that a measurement region of the tire in contact with the drum is moved in the circumferential direction of the tire.
When a load is applied to the pneumatic tire, the measurement region in contact with the drum deforms greatly. Meanwhile, a non-measurement region of the pneumatic tire which is not in contact with the drum deforms less. Thus, the vertical rigidity distribution of the tire generally depends on rigidity of each measurement region properly.
On the other hand, airless tires that include a cylindrical tread ring having a ground contact surface, a hub disposed radially inward of the of the tread ring and being fixable to an axle, and a spoke connecting the tread ring and the hub are known. Airless tires are also required to have excellent uniformity, same as pneumatic tires. Uniformity of completed airless tires may be measured similarly to pneumatic tires by mounting the hub to the measurement device.
Meanwhile, in airless tire, not only eccentric of the tread ring to the hub but also uniformity of the tread ring alone, i.e. vertical rigidity distribution of the tread ring alone in the circumferential direction is believed to affect the uniformity. It is therefore possible to further improve uniformity of airless tires if the vertical rigidity distribution of the tread ring alone is able to be measured.
When the vertical load is applied to the tread ring alone to measure the vertical rigidity of the measurement region of the tread ring which is in contact with the ground, the tread ring unfortunately deforms in an elliptical shape as the whole so that each part of the tread ring generates stress commensurately. For example, in the measurement region as well as the upward region of the measurement region, the compressive stress is generated on an outer peripheral side of the tread ring, and the tensile stress is generated on an inner circumferential side of the tread ring. On the other hand, in other regions, the tensile stress is generated on the outer peripheral side of the tread ring, and compressive stress is generated on the inner circumferential side. Therefore, not only stress generated in the measurement region which is in contact with the ground but also stress generated in the non-measurement region which is not in contact with the ground influences the measured values as the vertical rigidity.
Furthermore, since the technique that rotates a tread ring in the circumferential direction while applying vertical load onto the tread ring was not established, it has been therefore difficult to measure rigidity distribution of a whole length of the tread ring effectively. In particular, the technique that effectively measures rigidity distribution in a whole length of tread rings has been strongly desired in order to inspect rigidity of total or sampled tread rings in a manufacturing line of airless tires.